ECOSAFE project is focused on the control and safety issues raised by massive implementation of alternative fuel like hydrogen, especially in confined systems such as internal geometry of fuel cell stack. Understanding the conditions for propagation of a flame in such slender geometries requires the systematic study of the coupling processes between the flame shape, the flow, the thermal dissipation at the walls and the acoustics. For that purpose, laboratory experiments will be performed but pose a tremendous challenge in terms of visualizations. Complementary numerical simulations using an novel Reactive Lattice-Boltzmann Model will be a determining step for the project.

This project aims at comparing nonlinear evolution equations for premixed flames in large scale burners , where hydrodynamic effects are important, called self turbulent flames, to experiments. This type of flames is expected to have more applications in the next few years, as they have a number of advantages in terms of pollution reduction. However these flames are poorly understood. Building on recently developed experiments at the IRPHE laboratory that make use of a new device to damp acoustics and confirm the possibility of observing the propagation of wide flames through quiescent gases, we propose to experimentally study flames in simple situations such as a Hele-Shaw burner and test the predictions of theoretical models. The theoretical aspects will be developed in collaboration with G. Joulin (Pprime Institute), whose analytical works are a reference in the field. A better understanding of the coupling between turbulence and hydrodynamic instability of premixed flames is expected from this proposal. The study of flames in the Hele-Shaw burner will allow very precise experiments, leading to quasi two dimensional flames, which will be easy to study by image analysis. The results will be compared to theoretical and numerical studies of the Sivashinsky equation, the well-known model equation describing the non linear dynamics of premixed flames. We will study the statistical properties of flames in Hele Shaw cells, such as the probability density function of cells lengths or the fractal properties of the front. Analytical studies of the Sivashinsky equation will be performed, first in the case of a flame submitted to a stationary shear flow (where analytical solutions do not exist at the moment), then in the non stationary case. The study will be extended to very unstable flames submitted to an incident turbulence, with predictions of the turbulent flame velocity versus turbulence intensity. These predictions will be compared to experiments.

During embryonic development, cells must perform large-scale coordinated motion to shape functional tissues and organs. The development of computational models for such tissue flows may radically change the way we understand and control tissue morphogenesis. However, we still lack fundamental insights into the underlying mechanics of these processes. At the tissue level, these flows result from a complex interplay between growth, active stresses and complex mechanical properties. The previously overlooked role of mechanical properties, also referred to as rheology, has recently received growing attention as rheology has been shown to play a key role in various morphogenetic events. Yet, accurate measurements of tissue rheology in conditions relevant to morphogenesis remain currently challenging. Moreover, the general role of tissue rheology and its coupling to growth and active stresses during morphogenesis remains unclear. In FluidEmbryo, we aim to leverage recent advances in machine learning and computational fluid dynamics (CFD) to determine how complex mechanical properties of embryonic tissues can drive tissue flows shaping organs, with two objectives: - Objective 1 aims to build effective mechanical models for embryonic tissues. We will infer rheological properties and active stresses in embryonic tissues from microfluidic experimental data using physics-informed neural networks (PINNs). - Objective 2 aims to determine how mechanical properties can sculpt tissues. We will use high performance CFD simulations coupling complex rheology, growth and active stresses, to (i) identify morphogenetic processes that can be driven by tissue rheology in generic configurations and (ii) determine the role of tissue rheology during the axis formation of embryonic organoids. These two objectives will be pursued in close collaboration with two experimental groups with internationally recognized expertise in tissue morphogenesis and embryonic organoids. As a continuation of this project, our mechanical models will later be coupled with data-driven biochemical models, opening the way to multi-physics computational simulations for tissue engineering.

The dynamics of long flexible fibres in a turbulent flow involves multiple space and time scales and is the result of a complex interplay between their displacement, their deformation, and their interactions. Macroscopic fibres are present in several applications, such as the study of marine pollution by microplastics, the formation of long bacterial blooms in the oceans, and the flocculation of cellulose fibres in the paper industry. The dynamics of such elongated particles give rise to significantly more challenging questions than infinitesimal objects and are today at the centre of a strong interest in the experimental, numerical, and theoretical fluid dynamics communities. We focus in this project on long, thin, flexible fibres suspended in a turbulent medium. Our aim is to understand and model the fragmentation and aggregation processes that they experience. We will bridge three levels of description. From a microscopic viewpoint, we will address how the fibers coupling with the surrounding viscous fluid affect internal stresses and interactions between several filaments. This will allow us to improve coarse-grained mesoscopic models to account for break-ups, contacts, knots, and entanglements, both within a single fibre and between several of them. This step will feed the development of macroscopic descriptions, which, in tandem with filtered turbulence models, will allow the study of long-term global evolutions in practical settings where inhomogeneities or anisotropies play a crucial role. Our plan of action is organized into three axes. They combine the outcome of laboratory experiments, numerical simulations, and mathematical modelling, which reflects the specific expertise of the three partners. The first part focuses on fragmentation processes, with the objective of answering several questions: How do fibres with sizes in the turbulent inertial range break up? What is the effect of their own inertia, and in particular of violent inertial waves? How to account for plastic effects? The second aspect concerns the formation of fibre aggregates. Key questions that we will address are: How to describe the topology of knots and contacts from a statistical viewpoint? What are the mechanisms leading to aggregates? Do their size, shape, structure display universal properties? How is this changed when the fibers are active? The third line of attack consists in developing large-scale models of turbulent fibre suspensions. Several issues will be tackled: How to design stochastic Lagrangian models of long objects that cope with the intricate turbulent space-time correlations? Can size evolution be described by population dynamics models with both aggregation and breakup? Can entangled fibres be properly described as a porous, deformable objects with an effective dynamics? Experiments will be conducted at AMU in a newly built setup consisting of two arrays of high-speed jets that generate a homogeneous and isotropic turbulent flow inside a tank and in a rotor-strator cavity devoted to the study of anisotropic flows. High-speed cameras and a 3D reconstruction algorithm developed at AMU will be used to image the fibres. The numerical simulations will be based on a slender-body model of fibre and will consider two flow configurations: forced homogeneous isotropic turbulence and a bounded channel flow. The codes are available at UCA and are optimised for massive parallel servers. The mathematical modelling will rely on the expertise of the Inria team on stochastic analysis and population balance equations and will use in-house computational fluid dynamics (CFD) software.

The aim of this project is to gain new basic knowledge on the dynamics and instabilities of helical vortex systems, with relevance to applications involving flows around rotors. These include the wake generated by a helicopter and the flow behind a horizontal-axis wind turbine. In both cases, important fluid mechanical issues exist, related to either safety or efficiency of operation: the helicopter vortex wake is known to undergo a hazardous transition to a so-called Vortex Ring State in situations of steep descent, and the spatial evolution of a wind turbine wake has a direct influence on the performance of a second turbine placed downstream, which is today a common configuration. Despite a large amount of accumulated data, these phenomena are still poorly understood today, preventing major advances in these domains. The HELIX project focuses on the study of simplified generic vortex configurations to gain the missing physical understanding of helical vortex dynamics, which will complement the existing, mostly empirical knowledge. An original approach based on the intense use of theoretical modelling, combined with dedicated experiments and numerical simulations, will be developed in order to identify the main parameters governing the instabilities and transitions of helical vortex systems. The same strategy has already been applied with success by the project partners in the framework of previous European collaborations on aircraft trailing vortices, where significant new results on fundamental mechanisms of transition were obtained. The project also involves collaborations with the helicopter manufacturer EUROCOPTER and the Fluid Mechanics Group at the Technical University of Denmark, leading experts in wind turbine aerodynamics. They will assist the partners of the HELIX project in relating the fundamental results obtained using generic models to the full-scale applications. It is expected that this exchange will lead to new ideas and concepts for identifying and controlling rotor wake behaviour, which could help improve the safety of helicopter flight and the efficiency and lifetime of wind turbines.